Recently, a driving circuit of a display device, a personal computer or the like includes a semiconductor device such as a transistor, a diode or the like as a microscopic switching element. Especially in a display device, a semiconductor device is used as a selective transistor that supplies a voltage or a current in accordance with the gray scale of each pixel and is also used in a driving circuit that selects a pixel to which the voltage or the current is to be supplied. The characteristics required of a semiconductor device vary in accordance with the use thereof. For example, a semiconductor device used as a selective transistor is required to have a low off-current or little variance from other selective semiconductors. A semiconductor device used in a driving circuit is required to have a high on-current.
A semiconductor device including a channel formed of amorphous silicon, low-temperature polysilicon or single crystalline silicon has been conventionally developed for use in a display device as described above. The semiconductor device including a channel formed of amorphous silicon or low-temperature polysilicon is formed in a process at 600° C. or lower and therefore can be formed using a glass substrate. Especially, a semiconductor device including a channel formed of amorphous silicon can be formed with a simpler structure and in a process at 400° C. or lower, and therefore can be formed, for example, using a large glass substrate referred to as an eighth-generation glass substrate (2160×2460 mm). However, such a semiconductor device including a channel formed of amorphous silicon has low mobility and is not usable in a driving circuit.
A semiconductor device including a channel formed of low-temperature polysilicon or single crystalline silicon has higher mobility than the semiconductor device including a channel formed of amorphous silicon, and therefore is usable as a selective transistor and also in a driving circuit. However, such a semiconductor device including a channel formed of low-temperature polysilicon or single crystalline silicon has a complicated structure and needs a complicated process to be manufactured. In addition, such a semiconductor device needs to be formed in a process at 500° C. or higher, and therefore cannot be formed using a large glass substrate as described above. A semiconductor device including a channel formed of amorphous silicon, low-temperature polysilicon or single crystalline silicon has a high off-current. In the case where such a semiconductor device is used as a selective transistor, it is difficult to keep the applied voltage for a long time.
For the above-described reasons, a semiconductor device including a channel formed of an oxide semiconductor, instead of amorphous silicon, low-temperature polysilicon or single crystalline silicon, has recently been progressively developed (e.g., Japanese Laid-Open Patent Publication No. 2012-227521). It is known that a semiconductor device including a channel formed of an oxide semiconductor can be formed with a simple structure and in a low-temperature process similar to a semiconductor device including a channel formed of amorphous silicon, and has mobility higher than that of a semiconductor device including a channel formed of amorphous silicon. It is also known that such a semiconductor device including a channel formed of an oxide semiconductor has a very low off-current.
In a semiconductor device which uses an oxide semiconductor in a channel, a capacitor element having a MIM (Metal/Insulator/Metal) structure arranged with an insulation layer between a pair of metal layers which serve as an electrode is used in the case where a capacitor element is formed. It is necessary to form an electrode (metal layer) and a dielectric (insulation layer) for forming a capacitor element in order to form a capacitor element having a MIM structure. Therefore, the number of masks and the number of stacked thin films required for manufacturing a semiconductor device increases. As a result, manufacturing costs increase and manufacturing yield decreases.